Field of the Invention
The invention generally relates to handling a request for a Mobile Originated (MO) call, and more particularly, to apparatuses and methods for Access Domain Selection (ADS) during an ongoing IRAT (Inter-Radio Access Technology) procedure when the request for an MO call is received.
Description of the Related Art
With growing demand for ubiquitous computing and networking, various Radio Access Technologies (RATs) have been developed, such as the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, High Speed Packet Access (HSPA) technology, Long Term Evolution (LTE) technology, Time-Division LTE (TD-LTE) technology, LTE-Advanced (LTE-A) technology, and others.
Generally, a mobile phone only supports one RAT with one subscriber identity card for providing users with the flexibility of mobile communications at all times via the supported RAT using the single subscriber identity card. However, due to the complexity of various network deployments, so-called multimode mobile phones have been developed, which generally support two or more RATs and select one of the RATs for obtaining mobile services.
Since multimode mobile phones support multiple RATs, the Access Domain Selection (ADS) becomes an important issue for such mobile phones, especially when receiving a request for making a Mobile Originated (MO) call, such as a voice call or video call. For example, in second-generation (2G) networks (e.g., GSM/GPRS/EDGE networks) or third-generation (3G) networks (e.g., WCDMA networks), voice calls can only be made through the Circuit-Switched (CS) domain. In HSPA or LTE-based networks, voice or video calls can only be made through the IP Multimedia Subsystem (IMS) domain. That is, a multimode mobile phone is required to decide in which access domain the MO call should be made depending on the currently used RAT.
However, when an IRAT procedure is ongoing (i.e., when the multimode mobile phone is switching from one RAT to another, e.g., moving from an LTE network to a GSM network), the ADS may not be able to decide a proper access domain for the MO call due to the currently used RAT being indeterminate. As a result, the probability of the MO call being rejected by the network may increase if an improper access domain is selected.